This invention relates to an apparatus and method for detecting the concentration of nitrogen oxide as harmful components discharged from a variety of combustion devices, such as internal combustion engines.
Following discussions are given on the related art in the light of the present invention.
Up to now, there has been known an apparatus for detecting the concentration of nitrogen oxide in which, as disclosed for example in European Patent Publication 0678740A1, SAE paper No. 960334 p.137 to 142, 1996, a first measurement chamber communicating via a first diffusion rate defining layer with a gas to be measured and a second measurement chamber communicating via second diffusion rate defining layer with this first measurement chamber are formed by an oxygen ion conducting electrolyte layer, a first oxygen pumping cell and an oxygen concentration measurement cell are formed in the first measurement chamber by sandwiching a solid electrolyte layer between porous electrodes and, and in which a second oxygen pumping cell and an oxygen concentration measurement cell are formed in the second measurement chamber by sandwiching a solid electrolyte layer between porous electrodes, to constitute a sensor for detecting the concentration of nitrogen oxides (NOx) in the exhaust gases of the internal combustion engines.
In this type of the nitrogen oxide concentration detecting apparatus, the current is supplied to the first oxygen pumping cell so that the output voltage from the oxygen concentration measurement cell will be at a pre-set constant value and oxygen is pumped out from the first measurement chamber to control the oxygen concentration to a constant value. A constant voltage is applied across the second oxygen pumping cell for further pumping oxygen out of the second measurement chamber. The NOx concentration in the gas under measurement is detected from the value of the current flowing in this second measurement chamber.
In the exhaust gases from the internal combustion engines, that is gases under measurement, there are gaseous components other the NOx, namely oxygen, carbon monoxide or carbon dioxide etc. In the above-mentioned the nitrogen oxide concentration detecting apparatus, the current is caused to flow in the first oxygen pumping cell for extracting oxygen in the first measurement chamber, substantially in its entirety, and a constant voltage is applied across the second oxygen pumping cell in the second measurement chamber in a direction of pumping out oxygen out of the second measurement chamber, to decompose NOx in the gas under measurement into nitrogen and oxygen by the catalytic action of the porous electrode constituting the second oxygen pumping cell, to extract oxygen from the second measurement chamber. This enables detection of the NOx concentration in the gas under measurement without being affected by the gaseous components contained in the gas under measurement.
Also, in this type of the nitrogen oxide concentration detecting apparatus, it is necessary to heat the sensor to a pre-set activation temperature, e.g., not less than 800xc2x0 C., for correctly measuring the NOx concentration by the above-described detection method. Thus, a heater is separately provided for heating the sensor.
For controlling the sensor to a pre-set temperature using the heater, it may be contemplated to use a control method customarily used in an oxygen sensor configured for detecting the oxygen concentration in the exhaust gases using an oxygen concentration sensor comprised of a solid electrolyte layer sandwiched between porous electrodes.
That is, for controlling the sensor temperature, a variety of methods have been devised, such as a method controlling the amount of the current supplied to the heater so that the heat evolution in the heater will be constant, a method controlling the amount of the heater current by a pre-set control pattern so that the sensor temperature will be a target temperature, or a method of detecting the sensor temperature for controlling the amount of the current supplied to the heater, as disclosed in JP Patent Kokai JP-A-59-163556 or in JP Patent Kokai JP-A-59-214756. It may be envisaged to use these conventional control methods in the nitrogen oxide concentration detecting apparatus.
However, in the course of the investigation toward the present invention, certain problems have been encountered. That is in distinction from an oxygen sensor, the nitrogen oxide concentration detecting apparatus includes three cells in the sensor. Moreover, since it is difficult to provide a heater in each cell to control its temperature, there is posed a problem of how to apply the above-mentioned control methods.
In the nitrogen oxide concentration detecting apparatus, in particular, if the sensor temperature is changed, the oxygen concentration in the first measurement chamber, controlled by the current allowed to flow in the first oxygen pumping cell, and hence the oxygen concentration in the second measurement chamber, are varied, thus significantly varying the results of detection of the NOx concentration. Thus, for assuring detection accuracy of the NOx concentration, it is demanded that temperature control can be executed efficiently by a simplified structure so that the oxygen concentration in the first measurement chamber is not varied by changes in the sensor temperature. Up to now, a temperature controlling method capable of sufficiently meeting to this request has not been established, such that a nitrogen oxide concentration detection device which has realized temperature characteristics by a simplified structure has not been put to practical use.
Also, the nitrogen oxide concentration detection device suffers from a defect that, since the detected results are varied significantly by changes in the sensor temperature, the results of detection can be affected by changes in temperature of the gas being measured, such that, if the sensor temperature can be controlled accurately, the results of detection tend to be momentarily fluctuated during the transient period when the temperature of the gas being measured is changed.
In view of the above problems, it is a primary object of the present invention to provide a novel nitrogen oxide concentration detection apparatus (or method) capable of efficiently controlling the sensor temperature such as to prevent the results of detection of the NOx concentration from being varied.
It is a second object of the present invention to provide a novel nitrogen oxide concentration detection apparatus (or method) for correcting NOx concentration detection errors accompanying temperature changes of the gas under measurement that cannot be controlled by this temperature control.
Further objects of the present invention will become apparent in the entire disclosure.
According to a first aspect of the present invention, there is provided a nitrogen oxide concentration detection apparatus generally including a main body portion of a sensor having a first measurement chamber and a second measurement chamber. The first measurement chamber has a first oxygen pumping cell and an oxygen concentration measurement cell, the first oxygen pumping cell being made up of an oxygen ion conducting solid electrolyte layer sandwiched between porous electrodes. The first measurement chamber communicates via a first diffusion rate defining layer with a gas under measurement. The second measurement chamber is made up of an oxygen ion conducting solid electrolyte layer sandwiched between porous electrodes, and communicates with the first measurement chamber via a second diffusion rate defining layer. The nitrogen oxide concentration detection apparatus also includes pump current controlling means for pumping out oxygen from the first measurement chamber by the first oxygen pumping cell so that an output voltage of the oxygen concentration measurement cell will be constant, for thereby controlling the oxygen concentration in the first measurement chamber to a constant value. The nitrogen oxide concentration detection apparatus also includes constant voltage applying means for applying a constant voltage to the second oxygen pumping cell in a direction of pumping out oxygen from the second oxygen pumping cell, nitrogen oxide concentration detection means for detecting the concentration of nitrogen oxides in the gas under measurement based on the current value flowing in the second oxygen pumping cell on application of the constant voltage and a heater for heating the main body portion of the sensor to a temperature capable of detecting the nitrogen oxide concentration.
As characteristic of the present invention, the nitrogen oxide concentration detection apparatus further includes temperature detection means for detecting the temperature of the oxygen concentration measurement cell and heater current controlling means for controlling the current supply to the heater so that the temperature of the oxygen concentration measurement cell detected by the temperature detection means will be a pre-set target temperature.
According to a second aspect, the nitrogen oxide concentration detection apparatus further includes correction means for correcting the concentration of nitrogen oxide as detected by the nitrogen oxide concentration detection means depending on deviation from the target temperature of the temperature of the oxygen concentration measurement cell detected by the temperature detection means to temperature-compensate a detected value of the concentration of nitrogen oxides.
According to a further aspect, there is provided a novel method for detecting concentration of nitrogen oxide.
The method comprises various steps:
(a) providing a sensor having a first measurement chamber and a second measurement chamber, said first measurement chamber including a first oxygen pumping cell and an oxygen concentration measurement cell, said first measurement chamber communicating via a first diffusion rate defining layer with a gas under measurement, said second measurement chamber communicating with said first measurement chamber via a second diffusion rate defining layer;
(b) controlling pump current for pumping out oxygen from the first measurement chamber by the first oxygen pumping cell to control the oxygen concentration in the first measurement chamber;
(c) applying a voltage to the second oxygen pumping cell for pumping out oxygen from the second oxygen pumping cell;
(d) detecting the concentration of nitrogen oxides in the gas under measurement based on the current value flowing in the second oxygen pumping cell; and
(e) heating the sensor to a temperature capable of detecting the nitrogen oxide concentration;
The method further comprises the steps of:
(f) detecting the temperature of the oxygen concentration measurement cell; and
(g) controlling the temperature of the oxygen concentration measurement cell based on said temperature to a pre-set target temperature.
In the method, the concentration of nitrogen oxide as detected by step (d) is corrected depending on deviation from said target temperature of the temperature of the oxygen concentration measurement cell detected by step (f) to temperature-compensate a detected value of the concentration of nitrogen oxides.
The method comprises further features which will become apparent in the entire disclosure and claims.
The nitrogen oxide concentration detection apparatus detects the NOx concentration using a main body portion of the sensor having a first measurement chamber and a second measurement chamber. The first measurement chamber includes a first oxygen pumping cell and an oxygen concentration measurement cell. The first oxygen pumping cell is made up of an oxygen ion conducting solid electrolyte layer sandwiched between porous electrodes. The first measurement chamber communicates via a first diffusion rate defining layer with a gas under measurement. The second measurement chamber is made up of an oxygen ion conducting solid electrolyte layer sandwiched between porous electrodes, the second measurement chamber communicating with the first measurement chamber via a second diffusion rate defining layer. The pump current control means pumps out oxygen from the first measurement chamber by the first oxygen pumping cell so that an output voltage of the oxygen concentration measurement cell will be at a pre-set constant value for thereby controlling the oxygen concentration in the first measurement chamber to a constant value. The constant voltage applying means applies a constant voltage across the first oxygen pumping cell in a direction of pumping out oxygen from the first measurement chamber for controlling the oxygen concentration in the first measurement chamber to a constant value. The nitrogen oxide concentration detection means detects the NOx concentration in the gas under measurement based on the value of the current flowing in the second oxygen pumping cell on application of the constant voltage.
In the nitrogen oxide concentration detection device of the present invention, a heater for heating the main body portion of the sensor to a temperature capable of detecting the NOx concentration is provided as in the conventional device. The current supply to this heater is controlled by the heater current supply control means so that the temperature of the oxygen concentration measurement cell detected by the temperature detection means will be equal to a pre-set target temperature.
Thus, with the nitrogen oxide concentration detection apparatus of the present invention, it is possible to correctly detect the oxygen concentration in the first measurement chamber most significantly influencing the NOx concentration by the oxygen concentration measurement cell. Also, if, as in the present invention, the temperature of the oxygen concentration measurement cell is controlled to the target temperature by the heater, the temperature of the first oxygen pumping cell and the second oxygen pumping cell can be controlled to substantially a constant value. The result is that, according to the present invention, the oxygen concentration in the first measurement chamber can be controlled to a constant concentration by controlling the current supply to the first oxygen pumping cell by the pump current control means, thus enabling correct detection of the NOx concentration in the gas under measurement from the current flowing in the second oxygen pumping cell.
In the nitrogen oxide concentration detection apparatus of the present invention, the amount of the current flowing to the heater for heating the main body portion of the sensor is controlled so that the temperature in the oxygen concentration measurement cell for detecting the oxygen concentration in the first measurement chamber most strongly influencing the NOx concentration detection accuracy will be equal to the target temperature for correctly detecting the NOx concentration without being affected by changes in temperature of the main body portion of the sensor. Since the temperature control system for the main body portion of the sensor can be constituted simply by providing temperature detection means for detecting the temperature of the oxygen concentration measurement cell and heater current supply control means for controlling current supply to the heater, the nitrogen oxide concentration detection device of the present invention can perform temperature control of the main body portion of the sensor efficiently by a simplified structure.
In the nitrogen oxide concentration detection apparatus according to the second aspect, correction means corrects the concentration of nitrogen oxide as detected by the nitrogen oxide concentration detection means depending on an offset from the target temperature of the temperature of the oxygen concentration measurement cell detected by the temperature detection means for temperature-compensating a detected value of the concentration of nitrogen oxides. Therefore, with the present aspect, if the temperature of the main body portion of the sensor cannot be controlled to be equal to the target temperature despite the fact that the heater current supply control is being carried out by the heater current control means, the results of detection of the NOx concentration can be compensated for temperature by the correction means for further improving the detection accuracy of the NOx concentration.
Specifically, should the gas under measurement be changed suddenly in temperature, it may be an occurrence that the temperature of the main body portion of the sensor cannot be sufficiently controlled by temperature control by the heater current supply control means such that the temperature of the main body portion of the sensor is momentarily changed with changes in temperature of the gas under measurement. According to the present invention, the NOx concentration can be detected accurately even in such case thus further improving detection accuracy of the NOx concentration.
Although the temperature detection means for detecting the temperature of the oxygen concentration measurement cell may be implemented by providing a temperature-sensor device in the vicinity of the oxygen concentration measurement cell, the main body portion of the sensor becomes complex in this case. Moreover, it is difficult to measure the oxygen concentration measurement cell itself accurately.
It is therefore desirable to design the temperature sensor mans for detecting the internal resistance of the oxygen concentration measurement cell and to control the current supply to the heater so that the detected internal resistance of the oxygen concentration measurement cell will be of a (e.g., pre-set) value corresponding to the target temperature.
That is, the internal resistance of the oxygen concentration measurement cell is changed with the temperature of the oxygen concentration measurement cell (the internal resistance becomes lower the higher becomes the temperature), so that, if the internal resistance of the oxygen concentration measurement cell is detected as follows according to a third aspect. The temperature of the oxygen concentration measurement cell can be accurately detected from the detected internal resistance, without the necessity of providing a temperature-detection device in the main body portion of the sensor, thus enabling temperature control of the main body portion of the sensor to be carried out more easily and accurately.
If the internal resistance of the oxygen concentration measurement cell is detected in this manner by temperature detection means. The temperature detection means may be such means as either (a) applying a constant voltage for internal resistance detection across the oxygen concentration measurement cell for detecting the intensity of the current flowing at this time in the oxygen concentration measurement cell, (b) or such means as flowing a constant current for internal resistance detection across the oxygen concentration measurement cell thereupon to detect the voltage across both terminals of the oxygen concentration measurement cell.
However, for detecting the internal resistance of the oxygen concentration measurement cell in this manner, it is necessary to momentarily disconnect the pump current control means from the oxygen concentration measurement cell to stop current supply control for the first oxygen pumping cell by the pump current control means. That is, if the current is supplied to the oxygen concentration measurement cell for detecting the internal resistance, the voltage across both ends of the cell becomes non-coincident with the oxygen concentration in the first measurement chamber, such that the control operation of the pump current control means, if continued, leads to erroneous control of the oxygen concentration in the first measurement chamber. Thus, for evading this erroneous control, the control operation by the pump current control means is desirably discontinued for possibly evading this erroneous control.
The oxygen concentration measurement cell is for detecting the oxygen concentration in the first measurement chamber. Among the paired porous electrodes making up the cell, one electrode that is not contacted with the first measurement chamber needs to be of a constant value. To this end, a reference gas of a constant oxygen concentration, such as atmospheric air, can be introduced into this electrode side. However, if this reference gas is introduced in this manner from outside, it is necessary to provide a gap (conduit) in the main body portion of the sensor for introducing the reference gas, thus complicating the structure of the main body portion of the sensor.
According to a fourth aspect, for setting the reference oxygen concentration on the side of the porous electrode opposite to the first measurement chamber of the oxygen concentration measurement cell, it suffices if the porous electrode of the oxygen concentration measurement cell on the opposite side with respect to the first measurement chamber in the main body portion of the sensor is closed, such that a portion of oxygen in the closed space can leak to outside via a leakage resistance. The pump current controlling means causes the small current to flow in the oxygen concentration measurement cell in a direction of pumping out oxygen in the first measurement chamber into the closed space to control the amount of the current flowing in the first oxygen pumping cell so that the electromotive force generated in the oxygen concentration measurement cell will be of a constant value, while the closed spacing functions as an internal oxygen reference source. By so doing, there is not necessity of providing a gap (conduit) for introducing the reference gas into the main body portion of the sensor thus simplifying the structure of the main body portion of the sensor.
For measuring the internal resistance of the oxygen concentration measurement cell by temperature detection means, it is desirable if, as in the fourth aspect, the temperature controlling means periodically interrupts connection between the pump current controlling means and the oxygen concentration measurement cell and causes a current for the internal resistance detection larger than the small current to flow in the oxygen concentration measurement cell in an opposite direction with respect to the small current for detecting the internal resistance of the oxygen concentration measurement cell based on the voltage produced at this time across the electrodes of the oxygen concentration measurement cell.
That is, although the current for detecting the internal resistance can be caused to flow in the same direction as the small current, the current can be caused to flow in the opposite direction to the usual current flowing direction for suppressing electrode deterioration for manifesting the effect of periodically activating the electrode.
Meanwhile, if the current is caused to flow in the oxygen concentration measurement cell, the voltage generated by the oxygen concentration measurement cell is changed not only by the internal resistance of the oxygen concentration measurement cell, but by the oxygen concentration ratio between the electrodes. However, since the values of the oxygen concentration of both electrode sides of the oxygen concentration measurement cell (that is the ratio of the oxygen concentration in the first measurement chamber to that in the closed spacing) remains substantially constant by supplying the small current and by performing current supply control for the first oxygen pumping cell by the pump current control means, the electromotive force remains substantially constant during the supply period of the current for detecting the internal resistance. Besides, the back electromotive force generated by the current for the internal resistance detection is relatively large as compared with the variation in the electromotive force of the oxygen concentration measurement cell. Thus, according to the present invention, the internal resistance of the oxygen concentration measurement cell can be detected without being affected by changes in the electromotive force.
If the current for detecting the internal resistance is caused to flow in the oxygen concentration measurement cell in this manner through the oxygen concentration measurement cell, since the oxygen concentration measurement cell is changed in its direction of polarization in the solid electrolyte or between the solid electrolyte and the electrode, such that the electromotive force cannot be developed immediately depending on the ratio between the oxygen concentration in the first measurement chamber and that in the closed space in the same way as before measurement, thus giving a slightly offset value of the electromotive force. Therefore, it takes some period of time until this polarization is annulled to enable the oxygen concentration measurement cell to accurately detect the oxygen concentration in the first measurement chamber, such that, even if the control operation of the pump current control means is re-initiated directly following detection of the internal resistance, the NOx concentration cannot be detected accurately.
According to a fifth aspect, the following consideration is added.
For shortening the period of time which elapses after detection of the internal resistance until realization of correct measurement of the NOx concentration (non-detecting time), it suffices if the temperature detection means causing the current for internal resistance detection to flow in the oxygen concentration measurement cell for detecting the internal resistance, the temperature detection means subsequently causing a current larger than the small current to flow in the oxygen concentration measurement cell in an opposite direction to the current for detecting the internal resistance.
That is, if the current for detecting the internal resistance is caused to flow alternately in pulsed fashion through the oxygen concentration measurement cells, the oxygen concentration in each electrode side of the oxygen concentration measurement cell can be quickly reset to the stable state prevailing prior to detection of the internal resistance, thus shortening the period of time which elapses after detection of the internal resistance until realization of correct measurement of the NOx concentration.
According to the present invention, the temperature of one of the three cells of the main body portion of the sensor, i.e., one detecting the oxygen concentration in the first measurement chamber, which one most significantly influences the detecting accuracy of the NOx concentration, is detected as being the temperature of the main body portion of the cell to control the current supplied to the heater. However, it may be an occurrence that, depending on the structure of the main body portion of the cell, the temperature of the first or second oxygen pumping cell cannot be controlled close to the target temperature.
For realizing the effect of temperature control of the present invention more satisfactorily, in the main body portion of the sensor, the first oxygen pumping cell, oxygen concentration measurement cell and the second oxygen pumping cell are formed as respective different sheet-shaped solid electrolytes, whilst the first embodiment chamber and the second measurement chamber are formed by laminating the solid electrolyte layers with a gap between neighboring layers so that the solid electrolyte layers formed with the first oxygen pumping cell and the second oxygen pumping cell face outwards. The heater is made up of two sheet-shaped heater substrates with heater wire patterns formed thereon. The heater substrate is arranged with a pre-set gap between the neighboring layers on both sides of the laminating direction of the solid electrolyte layers in the main body portion of the sensor for heating the main body portion of the sensor. Preferably, the first diffusion layer is formed at a portion of the solid electrolyte layer provided with the first oxygen pumping cell, this portion including an area opposing the mid portion of the heater wire patterns of the heater substrates.
That is, if the main body portion of the sensor and the heaters are designed as described above, the solid electrolyte layer, provided with the oxygen concentration measurement cell, is sandwiched between the solid electrolyte layers provided with the first oxygen pumping cell and the second oxygen pumping cell, and heater substrates are arranged on both side in the laminating direction, so that if the temperature of the oxygen concentration measurement cell is controlled to be the target temperature by the current supply control to the heater, the first and oxygen pumping cells can be controlled more reliably to the target temperature. Moreover, the gas under measurement, following from the first diffusion layer and the second diffusion layer, can be heated sufficiently by the heater.
The result is that, according to a sixth aspect of the present invention, temperature fluctuations of respective cells of the main body portion of the cell become harder to occur, whilst the cells become difficult to be affected by the temperature of the gas under measurement, thus further improving NOx concentration detection accuracy.
According to a seventh aspect of the present invention, if, in this case, second diffusion rate defining layer is formed for being partially registered (overlapped) with at least a portion of the first diffusing speed determining layer when the main body portion of the sensor is projected from the layering direction of the solid electrolyte layers, and the oxygen concentration measurement cell is arranged in the vicinity of the second diffusion rate defining layer, the temperature of the main body portion of the sensor and the gas under measurement can be controlled more reliably to the target temperature, thus improving the NOx concentration detection accuracy.